Air springs are mounted between the axle and the chassis as supporting parts in a vehicle. The distance between the chassis and the axle should not change even for different loading conditions. For this reason, it is necessary to measure the air spring height and to readjust the system by pumping and venting air when there are deviations. An air spring control is also necessary for multi-axle vehicles for optimally utilizing the axle loads.
The invention relates to an air spring wherein the particular traveling elevation is determined with the aid of an ultrasonic running-time measurement within the air spring while utilizing at least one reference distance.
An ultrasonic distance measurement is utilized, for example, for the fill level measurements of vessels, to measure rooms in a building, to measure distance when parking a motor vehicle and to measure distance in autofocusing photo apparatus et cetera.
German patent publication 3,423,602 discloses an arrangement for measuring the distance between the chassis and the axle of a vehicle while utilizing an ultrasonic measurement system configured as a transmitter/receiver. The advantages of using ultrasonic sound within an air spring is that there is no turbulence of the sound wave within the air spring chamber because of the driving wind and the overpressure, which is present in the air spring, makes a very good range of the ultrasonic waves possible even at higher frequencies.
On the other hand, for distance measurement within the air spring chamber, the problem is present that pressure differences between 0 and approximately 20 bar and temperature ranges between -40.degree. C. and +120.degree. C. must be mastered. The speed of sound in a real gas (here, pumped-in air) is dependent to a great extent on temperature and to a lesser extent on pressure. For this reason, significant errors occur when making the distance computation based on a fixed pregiven sonic velocity.
To avoid errors, U.S. Pat. No. 4,798,369 suggests a compensation of pressure and temperature dependency by means of a computer circuit. However, how this is to be done is not disclosed in detail.
German patent publication 3,620,957 discloses an air spring having an ultrasonic pulse/echo system for making elevation measurements. An additional fixed target is suggested in order to cancel the effects of running speed changes of the pulse which can occur because of changes in air pressure, temperature and humidity within the air spring. In this way, a relative value of the running times is determined. A knowledge of the instantaneous sonic velocity is not required for computing the driving elevation.
The disadvantage in such a relative method while utilizing a reference distance (as known in a comparable manner from a telescope shock absorber disclosed in German patent publication G 87 02 817.4) is especially that the significant quantities of pressure and temperature cannot be explicitly indicated. German patent publication G 87 02 817.4 only suggests that thermistors be installed for temperature compensation. Here too, details are not presented.
The spring pressure, which is of interest for the loading state of the vehicle (the wheel load et cetera), cannot be determined with any of the above-mentioned air spring measurement arrangements.
Air spring systems are in use wherein separate pressure sensors operating on a DMS (resistance strain gauge) basis or piezo transducers are provided to measure pressure within the air spring volume. With these sensors, mechanical deformations of a membrane cause a change of resistance of the applied resistance elements or a displacement of electrical charges.
U.S. patent application Ser. No. 09/006,442, filed Jan. 13, 1998, discloses an arrangement for making contactless distance and pressure measurements within an air spring. Here, an ultrasonic arrangement comprises a transmitter/receiver component mounted on the chassis and a first reflector for forming a first reference distance s.sub.r1 and a reflector fixedly mounted on the axle 4 defining a measuring distance s.sub.r2. The transmitter/receiver component with the first reflector is elastically mounted in a pipe stub fixed to the chassis. A second reflector is fixedly mounted on the pipe stub and defines a second reference distance s.sub.r2.
By comparing the running times, which are assigned to the two reference distances as s.sub.r1 and s.sub.r2, the displacement of the elastically mounted sonic transducer is obtained and the internal pressure in the air spring can be determined.
The yielding of the elastic suspension constitutes a decisive factor for determining the pressure. For this reason, a high precision is required with reference to maintaining the geometric dimensions of the acoustic position as well as its embedment. The surface of the elastic mounting should always be kept clean to avoid a negative effect on the elasticity of the embedment by dirt. No investigative results are presented with reference to the resistance to deterioration of the elasticity module.